1. Field of the Invention
The present invention generally relates to a method of cleaning a substrate having a patterned metal layer formed thereon and in particular to a cleaning method and a cleaning solution for removing metallic contaminants adhering to the substrate on which a metal layer has been formed by fine and high-density patterning.
2. Description of the Related Art
In recent years, as devices are becoming denser, substrates having thin-film multilayered structure, such as a semiconductor substrate, a liquid crystal substrate, a printed substrate for electronic devices and a substrate having magnetic materials, are subjected to fine processing of the multilayered films.
In particular, semiconductor devices become highly dense and gate electrodes thereof are made fine. For this, low-resistant suicide materials (WSix, TiSix) or metal materials such as tungsten (W) etc., or multilayered structure having TiN, WN sandwiched between W or WSix film or TiSix film came to be used as a gate electrode material.
FIG. 1 shows the schematic structure of a thin-film transistor. After a gate oxide film (SiO.sub.2) 2 is formed on a Si (silicon) substrate 1, a film of metal is deposited on the gate oxide film 2. The film of metal is finely patterned by dry etching to form a gate electrode 3. Thereafter, a source region 4 and a drain region 5 are formed in the substrate 1 by ion implantation.
In these steps, metallic contaminants 6 such as Fe. Al etc. are generated from the dry etching apparatus and the ion implanter and adhere to the surface of the gate oxide film 2. The metallic contaminants 6 on the gate oxide film 2 diffuse into the Si substrate 1 through the gate oxide film 2 in a subsequent thermal treatment step. This causes leak current to occur readily in the channel region under the gate electrode 3, or to reduce the maximum permissible voltage between the source region and the drain region, resulting in deterioration in the reliability of the device.
Accordingly, it is necessary to remove the metallic contaminants 6 from the surface of the gate oxide film 2, and after dry etching of the film of metal on the gate oxide film 2 and after ion implantation, a cleaning treatment using an acidic cleaning solution is applied so that the metallic contaminants 6 developed during these steps are removed from the surface of the gate oxide film 2.
Usually, In the semiconductor process using a Si substrate, SPM (sulfuric acid-hydrogen peroxide mixture) or HPM (hydrochloric acid-hydrogen peroxide-water mixture) is mainly used as a cleaning solution for removing metallic contaminants.
Liquid crystals and printed substrates are rarely cleaned with an acidic cleaning solution and are cleaned with an organic solution such as IPA (isopropyl alcohol).
However, the conventional acidic cleaning solutions (SPM, HPM) used for removal of metallic contaminants have the problem of their possible etching of materials constituting the gate electrode 3.
In particular, TiN and W among metal wiring materials used as low-resistance gate are easily etched with these acidic solutions, so these cannot be used for cleaning treatment.
As shown in FIG. 2A, for example, the gate electrode 3 is a DRAM gate electrode composed of multilayered films such that a poly-sillcon layer 7 is formed on the gate oxide film 2 and a silicide (Tiix) layer 8 and a nitride film (TiN) 9 are sequentially deposited on the poly-silicon layer 7. If the cleaning solution SPM or HPM is used for removal of metallic contaminants developed after dry etching or after ion implantation, then the side wall of the nitride film (TiN) 8 is etched as shown in FIG. 2B, so the multilayered structure cannot be maintained and the gate electrode and the wiring may be removed.
Further, when DHF (fluoric acid-aqueous system-water mixture) effective for removal of metals is used as the cleaning solution, the silicide layer (TiSix) 9 is greatly etched and the gate oxide film (SiO.sub.2) 2 is also etched, so DHF cannot be used.
As described above, as the integration of semiconductor devices advances, the width of the gate electrode is also made fine, so the amount of material etched by treatment with the conventional acidic cleaning solution cannot be neglected, and there was need for a cleaning solution capable of removing metal contamination without etching low-resistance metal materials.